How do you clean spilled liquid detergent on the fabric seat ?

take the seat out and drown it with water- clean seat also.

If you can't get the seat out, you still need to drown the seat, but do it a little at a time with a sponge. I hope not a lot was spilled because a capful is a lot. If you have or can rent a wet/dry shop vac, it will be a little easier. Every time you think you have it all out, scrub with a sponge or a cleaning brush. If you see bubbles or white stains, it's not out. The seat will need to dry thoroughly, even if you have to cover it with towels for temporary use. When it is completely clean and dry, spray it well with Scotch Guard. Best wishes 😇

Try using a shop vac